(i) Field of the Invention
The present invention relates to an oxygen sensor disposed before an exhaust gas purifying catalyst of an engine which uses a hydrocarbon containing fuel with a ratio of hydrogen atoms and carbon atoms, i.e., H/C ratio of three or more.
(ii) Description of the Related Art
Various researches and proposals have been developed as conventional oxygen sensors for gasoline engines. For example, Examined Japanese Patent Publication No. Hei 8-7177 discloses an oxygen sensor which is superior in durability, can effectively use a noble-metal catalyst and which can keep stable an air/fuel ratio control for a long time without deviation from the stoichiometric point or deterioration in responsiveness.
Specifically, the oxygen sensor is provided with a reference electrode disposed on a surface of a solid electrolytic body having an oxygen ion conductivity, a detection electrode disposed on the other surface of the solid electrolytic body, a first porous protective layer disposed to cover the detection electrode, and a second porous protective layer disposed on the first protective layer. For example, the detection electrode is formed into a thickness of 0.9 .mu.m by chemical plating of platinum, the first protective layer is formed by plasma spray coating of spinel powder, and the second protective layer is formed by baking titania paste containing a noble metal catalyst. Additionally, the second protective layer contains 0.02 to 5 mol % of noble metal catalyst.
However, a preferable oxygen sensor has not been proposed which is disposed before or upstream from an exhaust gas purifying catalyst of an engine which uses CNG (compressed natural gas) fuel or another hydrocarbon containing fuel with a ratio of hydrogen atoms and carbon atoms, i.e., H/C ratio of three or more.
Moreover, for example, a CNG engine car contains a larger amount of hydrogen and methane in its exhaust gas than a gasoline engine car. Therefore, if the oxygen sensor for the gasoline engine is mounted upstream from the catalyst for purifying the exhaust gas of the CNG engine, and is continuously used for engine control, the under-mentioned problems (1) and (2) arise:
In this case, the oxygen sensor for the gasoline engine is provided with a 0.9 to 1 mm thick solid electrolyte of zirconia; a reference electrode and a detection electrode, each of a 0.9 .mu.m thick platinum layer, formed inside and outside the solid electrolyte; a 100 .mu.m thick first protective layer of spinel for covering the reference electrode; and a 50 .mu.m thick second protective layer of titania powder for covering the first protective layer. The catalyst of platinum is contained by 0.4 mol % in the entire second protective layer
(1) When the oxygen sensor is continuously used in the flow of exhaust gas, the catalyst in the second protective layer is sublimated and scattered, the capability of oxidizing hydrogen is deteriorated, engine control is deviated toward the lean side, and nitrogen oxides are increased.
(2) When the oxygen sensor is continuously used in the flow of exhaust gas, especially on operation condition that a large amount of methane is exhausted, e.g., on operation condition that a small load is applied and the number of revolutions is large, the circumstance is that engine control is deviated toward the lean side as in the above (1); nevertheless, the engine control is deviated toward the rich side.
The reasons for the above-mentioned (1) are as follows:
The exhaust gas from the CNG engine has a larger content of hydrogen than the gasoline engine in all regions. Therefore, when the capability of oxidizing hydrogen is decreased in the second protective layer, hydrogen passes through the first and second protective layers to reach the detection electrode. In this case, a hydrogen molecule is lighter than an oxygen molecule. Since the diffusion rate of the hydrogen molecule is larger than that of the oxygen molecule, the concentration of hydrogen is relatively higher than the concentration of oxygen around the detection electrode. Accordingly, the oxygen around the detection electrode is consumed to decrease the oxygen partial pressure. It is supposed that the control point of the oxygen sensor is deviated toward the lean side because of a diffusion difference of hydrogen and oxygen in the protective layers.
The reasons for the above-mentioned (2) are as follows:
By continuously using the oxygen sensor, the detection electrode metal is sintered or sublimated, the effective area of the electrode is reduced, and oxidation, i.e., burning reaction of a large amount of methane contained in the exhaust gas is insufficient. Although the exhaust gas is actually in a rich state, due to incomplete combustion, oxygen remains around the detection electrode, and the oxygen partial pressure is not lowered. As a result, the control point of the oxygen sensor is supposedly deviated to the rich side.
Moreover, as described in (2), although the protective layer is formed, the detection electrode metal is sintered or sublimated for the following reasons:
The sintering of the detection electrode metal progresses regardless of the presence of the protective layer mainly because it depends on oxygen concentration and temperature. On the other hand, sublimation depends on reducible gas, temperature and gas flow rate. Therefore, sublimation is related with the presence of the protective layer to some degree, but remarkably progresses if the catalyst capability of the protective layer is deteriorated. As aforementioned, the exhaust gas of CNG engine contains a large amount of hydrogen, but hydrogen has a strong reducing force. Therefore, if the capability of oxidizing hydrogen of the second protective layer is lowered at a high temperature and a high flow rate, the amount of hydrogen reaching the detection electrode is increased, and the detection electrode metal tends to easily sublime.